Heat-shrinkable plastic films are widely used in applications such as shrink packaging, shrink labels, cap seals and the like, due to their characteristic property of shrinking by heat. Among them, polyvinyl chloride, polystyrene, and polyester films are used for labeling, cap sealing, or collective packing various containers such as polyethylene terephthalate (PET), polyethylene, glass, and other containers.
However, polyvinyl chloride films carry problems that they are lower in heat resistance, generate hydrogen chloride gas during incineration, and give rise to dioxins. Alternatively, polystyrene films are poorer in solvent resistance and thus demand those inks having special ingredients for printing. In addition, these polyvinyl chloride and polystyrene films have a problem that when used as a shrink label for PET containers (PET bottles and the like), they should be separated from the containers if the containers are to be recycled and reused.
Carrying no such problems, polyester films are highly expected as a shrink label replacing the polyvinyl chloride and polystyrene films, and the amount of their consumption is gradually increasing parallel to the expansion of the amount of PET containers used.
However, further improvement is still needed in the shrinkage properties of heat-shrinkable polyester films. In particular, as the films tend to cause shrinkage shading and crinkling when they are used for wrapping and shrink around containers such as PET, polyethylene, glass, and other bottles, the characters and drawings printed on the films before shrinkage are sometimes deformed after wrapping and shrink. Accordingly, there exists a need from users for reducing the deformation as much as possible. Additionally, the shrinkage stress of the films is generally smaller, and thus the films sometimes do not tightly bind to containers. Further, heat-shrinkable polyester films are sometimes smaller in shrinkage at lower temperature compared to heat-shrinkable polystyrene or other films, and accordingly demand higher temperature for shrinkage to the desired degree. However, shrinkage at high temperature is often accompanied with deformation and whitening of the bottles.
Incidentally, for wrapping bottles by heat-shrinkable films, the heat-shrinkable films are hitherto first printed (in printing step) and then processed into the shape suitable for loading onto the containers [such as labels (tubular labels), tubes, bags, and the like]. Subsequently, these processed films are loaded onto the bottles, which are transferred on conveyer belt through a heating tunnel (shrinkage tunnel), and the films are tightly bound to the containers by heat shrinkage. Steam tunnels, wherein the films are allowed to shrink by blowing steam, and hot-air tunnels, wherein the films are shrunk by blowing heated air, and the like are commonly used as the shrinkage tunnels.
The steam tunnels are generally better in heat transfer efficiency than the hot-air tunnels, and thus allow more uniform heat shrinkage of such labels and provide shrunk products better in appearance. However, even if the steam tunnel is employed, the heat-shrinkable polyester films are often not quite satisfactory from the viewpoint of product appearance property after heat shrinkage, compared to polyvinyl chloride and polystyrene films.
Further, the hot-air tunnel has a tendency to cause larger variation in temperature during heat shrinkage than the steam tunnel. Thus, when polyester films, lower in the product appearance property after heat shrinkage than polyvinyl chloride and polystyrene films, are heat-shrunk therein, the resulting films often have whitening due to shrinkage, shrinkage shading, crinkling, deformation, and the like, and especially a problem in appearance due to whitening.
In addition, there exists a need for further improving the processability and printability of the heat-shrinkable films. One of the methods to improve the processability and printability would be to make the films more uniform in thickness. Proper adjustment of the uniformity in film thickness can prevent crinkling and meandering of the films during production and thus improve the processability of films. It also prevents the films from partial lack of print during printing. Therefore, it is quite important to improve the uniformity in film thickness of the heat-shrinkable films.
A method of electrostatically bringing the melt-extruded film into tighter contact with a casting roll during the film being cooled by the roll has been known as the method for improving the uniformity in film thickness. In order to make the film securely contact with the roll electrostatically, it is important to provide a large number of electric charge carriers immediately after extrusion and before contact with the roll on the surface of the melt-extruded film. For the purpose of providing many electric charge carriers, it is effective to reduce the resistivity of the polyester by modifying the polyester, and indeed many efforts have been made for that purpose. For example, Japanese Examined Patent Publication No. 3-54129 disclosed that the addition of a magnesium compound, a sodium or potassium compound, and a phosphorus compound during production of polyethylene terephthalate (PET), at a Mg atom concentration of 30 to 400 ppm and a Na or K atom concentration of 3.0 to 50 ppm, and at an atomic number ratio of Mg to P (Mg/P) of 1.2 to 20, reduced the resistivity of PET films.
In this patent, the magnesium compound was added at the point when the degree of esterification is 20 to 80%; the sodium or potassium compound, before the intrinsic viscosity of the polyester reaching 0.2; and the phosphorus compound, after the point when the degree of esterification reaches 90% or more and before the point when the intrinsic viscosity reaches 0.2. The addition in that order suppressed generation of insoluble foreign materials and improved the quality of the films.
An object of the present invention is to provide a heat-shrinkable polyester film roll from which container-wrapping films reliable in processability and printability may be cut off in high yield.